The Controller Area Network (CAN), as well as an extension of CAN referred to as “time triggered CAN” (TTCAN), are described, for example, in German Application No. DE 100 00 305. The media access control method used in CAN is based on bitwise arbitration. In bitwise arbitration, several subscriber stations can simultaneously transfer data via the channel of the bus system without thereby disrupting data transfer. The subscriber stations can furthermore, upon transmission of a bit via the channel, ascertain the logical state (0 or 1) of the channel. If a value of the transmitted bit does not correspond to the ascertained logical state of the channel, the subscriber station then terminates access to the channel. With CAN, bitwise arbitration is usually performed in an arbitration field within a message to be transferred via the channel. Once a subscriber station has completely transmitted the arbitration field onto the channel, it knows that it has exclusive access to the channel. The end of the transfer of the arbitration field thus corresponds to a beginning of an authorization interval within which the subscriber station can exclusively use the channel. According to the CAN protocol specification, other subscriber stations are not allowed to access the channel, i.e., transmit data onto the channel, until the transmitting subscriber station has transferred a checksum field (CRC field) of the message. A point in time at which transfer of the CRC field ends thus corresponds to an end of the authorization interval.
Bitwise arbitration results in nondestructive transfer of messages via the channel. The messages transferred on the CAN bus are also called “data frames.” Nondestructive transfer means that CAN has good real-time properties, whereas media access control methods in which the messages transmitted by one subscriber stations can be destroyed during transfer via the channel because of a collision with a further message transmitted by another station have appreciably less favorable real-time behavior, since a delay in data transfer occurs due to the collision and the retransmission of the message necessitated thereby.
The CAN protocols are suitable in particular for transferring short reports under real-time conditions. If larger data blocks are to be transferred via a CAN domain, the relatively slow bit rate of the channel then becomes a limiting factor. In order to ensure correct functioning of bitwise arbitration, a minimum duration that depends principally on the extent of the bus system, the signal propagation speed on the channel, and intrinsic processing times in the interface modules of the bus subscribers must be complied with during arbitration for the transfer of a bit, since all the bus subscribers must have a uniform picture of the bus state (0 or 1), and equal access to the bus state. The bit rate therefore cannot be arbitrarily increased by decreasing the duration of the individual bits. In order nevertheless to allow sufficiently fast transfer, via a communication interface that is actually provided for connection to a CAN domain, of a relatively large data block necessary for programming a control unit, German Application No. DE 101 53 085 proposes that for transfer of the data block, the communication interface be switched over temporarily into a different communication mode in which bitwise arbitration is not carried out and a relatively high bit rate is thus possible. Communication using the CAN protocols must, however, be interrupted for a certain time in this context. For example, if operation of the bus system according to the CAN protocols can no longer be effected because of an error, a failure of the bus system then occurs. In addition, the transfer of a relatively large data block results in a considerable delay in the subsequent transfers to be performed in accordance with CAN protocols, so that the real-time properties of the CAN system are impaired.
German Application No. DE 103 11 395 describes a system in which asynchronous, serial communication can be accomplished alternatively via an asymmetrical physical protocol or the symmetrical physical CAN protocol, and a higher data transfer rate or data transfer security for asynchronous communication is thereby attainable.
German Application No. DE 10 2007 051 657 proposes to utilize an asynchronous, fast, non-CAN-conforming data transfer in the exclusive time windows of the TTCAN protocol, in order to increase the volume of data transferred.
G. Cena and A. Valenzano, “Overclocking of controller area networks,” Electronics Letters, Vol. 35, No. 22, p. 1924 (1999), discusses in theoretical terms the effects of overclocking the bus frequency in sub-regions of the message on the effective data rate that is effectively achieved, but without going into details as to methodology and the various states and state transitions of the bus subscribers.
It is apparent from the documents cited that the existing art does not provide satisfactory results in every respect.